Just to follow up on my water leak. I found it and have fixed it (hopefully).
I removed all of the carpets, front and back and dried the interior floor. The carpets were soaking and needed drying out. I checked all of the drains with copious water from a hose but could find no issues and no water coming in.
I then used the hose to spay all of the front windscreen area and almost immediately had a trickle of water running down the soundproofing in the passenger footwell. On closer examination I could see it coming out from the edge of the Heater Motor housing near one of the mounting screws. All of the screws were tight so that wasn't a problem.
I removed the Heater Motor and then saw that the water was coming in at the lower windscreen trim. The photo below is taken from the passenger footwell, looking up through the aperture where the heater motor fits. In the picture the dark grey strip with the water drop is the lower screen trim piece. To the left of it is the actual windscreen, and to the right is the plastic scuttle below the screen.
I removed the trim but could see no obvious damage or corrosion, but it was quite dirty. I cleaned the lower edge of the screen and the trim, and put a bead of sealant in the trim on the windscreen side only and refitted it. Left it overnight to cure and then repeated the test. No more water.
Left car out in the rain for 4 days without the carpets and still no water. So I'm optimistic it's fixed.
@MWMan - I did check around my sunroof while doing the work but could not see any parts that resemble your photo.